I. Field of the Invention
This invention generally relates to the prevention of migration of basis metal to overplate.
II. Description of the Prior Art
Deposition of gold on copper and its alloys can be accomplished satisfactorily by plating a diffusion barrier coating between the basis metal and the overplate. Without the barrier, copper in the basis metal dissolves in the gold layer and quickly migrates to the surface, even at room temperatures. On exposure to air, the copper atoms on the surface of gold layer can be easily oxidized to form black oxides. Many consumer articles such as watch bracelets, watch cases, imitation jewellery and spectacle frames are plated with gold. The presence of these black copper oxides destroys the aesthetic appearance of gold coating used as decorative purpose.
A number of materials are known for forming diffusion barriers for copper. They include Ni, Co, Pd, W, Mo and other high melting points metals. These materials can be deposited singly or co-deposited on copper by conventional methods such as electroplating, electroless deposition, physical vapor deposition (PVD) or chemical vapor deposition (CVP). This coating is initially deposited on the copper/copper alloy basis metal intended as diffusion barrier coating. The decorative gold overplate is then plated on the diffusion barrier, achieving the goal to impede the migration of copper atoms to the gold plating.
Ni has been used extensively as the diffusion barrier material for copper for the manufacturing of consumer products. However, Ni suffers the drawback on its relative case of corrosion when used in consumers articles. These articles are worn with prolonged contact with the human skin. Perspiration secreted from the skin contains sodium chloride, among other components, deposits on the article during prolonged contact. The perspiration migrates through the pores of the gold overplate to the Ni under coating and corrodes the metallic Ni diffusion barrier to Ni(II) state. The nickel ion dissolves easily in the perspiration and migrates back to the outer gold coating of the article.
Ni(II) ion is known to irritate human skin and causes sensitization of humans skin to nickel, leading to allergic reactions (see for example, xe2x80x9cMetall als Allergenxe2x80x9d, R. Breitstadt; Galvanotechnik, vol. 47, no. 1; 1993; pp.-16-19). These findings revealed from detail studies on the allergic reactions on human skin (see for example, xe2x80x9cReinst-Palladium als Ersatz fur Palladium/Nickel. Einsatz fur Endschichten und als Diffusionssperrexe2x80x9d, K. -P. Beck, Glavanotechnik, vol. 47, no. 1; 1993; pp.20-22) have initiated the issuance of the Directive 76/769/EEC in 1994 controlling the use of Ni in consumer articles and the liberation of Ni(II) ions (see for example, xe2x80x9cControl of nickel emission in jewellery and related itemsxe2x80x9d, R. V. Green and J. F. Sargent, Transactions of the Institute of Metal Finishing, vol. 75, no.3; 1997; p. B51-52). In essence, metal objects with the intent for prolonged contact with human skin and are made of nickel-containing alloys or coated with nickel-containing substances, should not release nickel in excess of 0.5xcexcg/cm2/week. The specifications for monitoring the said release rate are documented in the standards, EN1811 and EN12471 adopted by the European Committee for Standardization (CEN) in late 1999.
Accordingly, the present invention describes a technique of utilizing a ternary alloy coating of Co, W and P deposited either with electroplating and electroless plating techniques to form an efficient barrier to reduce the migration of copper.
The present invention describes a technique of depositing CoWP coating with either electroplating or clectroless plating method on copper or copper alloy. Gold or gold alloy will subsequently plate on the ternary alloy coating. The purpose of the ternary alloy coating is to form a diffusion barrier reducing the migration of copper to the gold top coating. Coatings formed with either technique gives a mixture of amorphous-microcrystalline structure, thus enhancing both barrier and corrosion resistant properties. Electroplating furnishes a rapid technique in yielding an efficient coating. Electroless deposition process does not require the use of electric current. Good coverage is an additional benefit with this plating technique.